When there's no answer, again and again, at some point you have to stop calling. NASA announced Tuesday (May 24) that it will cease its daily attempts to contact Spirit, a robotic rover on Mars that went incommunicado last year.

"Planned communications will be done on May 25 but there may be some passive communication attempts after that," NASA spokesperson Veronica MacGregor told SPACE.com today.

Project manager John Callas of NASA's Jet Propulsion Laboratory told reporters today that the last commands will be sent up Wednesday, and orbiting spacecraft will listen for a return signal through the end of May. Spirit's electronics have likely become permanently damaged by Mars' harsh winter, however, and chances of a last-minute response are slim.

The decision to let the Mars rover Spirit rest in peace marks the official end of its successful six-year mission.

The rover far surpassed the space agency's expectations by functioning more than 20 times longer than predicted — its mission was planned to last three months — and driving 4.8 miles across the Martian surface, 10 times longer than planned. [Photos from Mars Rovers Spirit & Opportunity]

"In addition to the great exploration and the accomplished scientific discovery over its six active years, there's also a 'great intangible' that goes with it: Spirit has made Mars a familiar place," Callas said in a teleconference. "Mars is no longer this distant, mysterious location. Human beings now, every day, go to work on Mars ... Mars is now our neighborhood."

The Martian Spirit

Spirit and its twin rover, Opportunity, parachuted to opposite sides of the Red Planet in January 2004. According to Callas, their greatest achievement was uncovering geologic evidence that dry and dusty Mars was far wetter billions of years ago, and could even have had an ancient environment that may have been favorable for microbial life. [Go No More a-Rovin': Mars Science Station Spirit]

Spirit developed software trouble early on in the mission, but engineers on Earth managed to solve the glitch and keep the rover going. Other glitches, mostly due to the increasing age of the rover and rigors of exploring Mars popped up over time.

"We have developed a strong emotional attachment to both of these rovers. They are just the cutest darn things out in the solar system and I think the public joins us in that. They are beautiful, accomplished little proxies out on the surface of Mars and we're quite proud of them and have become quite attached to them," Callas said. "There's a sadness that we have to say goodbye to Spirit but we have to remember the great accomplishments and the blessings that we have received from having this rover operate for so long, six years. I imagine even Opportunity is a little bit sad to be alone and looking forward to the arrival of MSL."

MSL, or the Mars Science Laboratory, is NASA's next Mars rover. Named Curiosity, the robot is slated to launch toward the Red Planet in late November. Curiosity is larger than Spirit and Opportunity and carries a nuclear power source, making it the most ambitious Mars rover mission yet. [Vote Now! Where Should NASA's New Mars Rover Land?]

Memorial service

Spirit's final troubles began in April 2009, when it got stuck in a patch of Martian sand. Engineers worked for eight months attempting to free the rover, but to no avail. In its stationary position, Spirit's solar panels weren't able to tilt toward the sun and so it lost power during the winter of 2009 and 2010.

When Spirit lost power, its internal temperature plunged to minus 67 degrees Fahrenheit — the coldest it had ever experienced — and it likely sustained electronic damage that prevented it from powering back up last spring.

Mission managers had been considering scaling back the listening campaign from once a day to once a week, but on Monday, Callas notified the rover team that he decided against that plan, saying that any continued effort would cut into other missions.

A tale of two rovers

Of the twin rovers on Mars, Opportunity has had an easier go of it, landing in a shallow lake bed.

It remains functional to this day and has traversed more than 18 miles of the Martian surface. Spirit encountered difficulties from the beginning. It landed in a crater, had to scale out, and immediately faced technical problems that engineers struggled to correct from Earth.

Nonetheless, Spirit achieved plenty, scaling a mountain the height of the Statute of Liberty in 2005, and recording Martian dust devils as they formed for the first time, which NASA later made into movie clips. It also snapped the highest-resolution photo ever taken on another planet (a panoramic view of a Martian plain), investigated several craters and found hints of water history on Mars in the interior of a rock.

According to NASA's TV schedule, the agency plans to broadcast a program next Tuesday, May 31 at 2:30 p.m. titled "Mars Spirit Rover Celebration: An End to a New Beginning."

But don't expect a funeral for Spirit.

David Lavery, NASA's program executive with solar system exploration, said if anything the Mars rover science team will hold an Irish wake for the plucky robot, possibly in July.

"I think we'll all sit around and have a sip of Guinness and reminisce about when Spirit was a wee small little rover and look back at the accomplishments and successes rover had over its entire lifetime," Lavery said.